/Engine/Plugins/Online/OnlineSubsystem/Source/Public/Interfaces/OnlineSessionInterface.h
Registers a player with the online service as being part of the online session
true if the call succeeds, false otherwise
bool RegisterPlayer ( [FName](API\Runtime\Core\UObject\FName) SessionName, const [FUniqueNetId](API\Runtime\CoreOnline\Online\FUniqueNetId) & PlayerId, bool bWasInvited )